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On this 13th day of our Christmas Advent we thought you might like to think about your true love, and for that you need your maths.

You might find it surprising but mathematics provides some of the most profound metaphors for love and human relationships. From converging infinite sequences describing the attraction and force of love in A disappearing number to Elizabeth Barrett Browning's "How do I love thee, let me count the ways…", the rich language in mathematics provides just an accurate and evocative description of human emotions as it rigorously defines mathematical concepts.

The number 12 is very flexible. You can write it as 2 x 6 or as 3 x 4. Or, even better, as 2 x 2 x 3. Which brings us to those very special numbers called the primes: those numbers that are not divisible by any other number apart from 1 and themselves. Every number can be decomposed into a product of primes, for example 12 = 2 x 2 x 3. The primes are the atoms of number theory, they are also connected to one of the hardest open problems in maths, and even to the weird world of quantum physics.

As is tradition we start with a fact about this Carnival's number: 117. It's divisible by 3. That might not be so exciting to everyone, but to someone such as myself, as a mathematician who is notoriously bad at numbers and arithmetic (my friends think it's hilarious to ask me to divide the bill at a restaurant), being able to tell this quickly about any integer gives me a feeling of vast power over these numbers. You can tell if a number is divisible by 3 if the sum of the number's digits is divisible by 3: in our case, 1+1+7=9. Neat!

This trick gives me a great sense of power over the integers as it allows me to rule out vast swathes of them (well, a third) as not being prime, at a glance. Combine that with our instant knowledge that any even numbers, or those ending in 5 or 0 aren't prime, and you've really got a good of chance of being able to tell if any random number someone tells you is prime. There's other neat division rules too, for 7 and 11 and so on. But the really interesting thing is why they work… Can you explain why the division rule for three works? Give us your best explanations in the comments below!

Now to the Carnival. We had many great submissions, thank you to everyone who sent one in. The one that had me hooked for the longest was the wonderful Parable of the Polygons by Vi Hart and Nicky Case, submitted by Katie. This brilliant interactive post explores the maths of diversity based on the work of Nobel Prize-winning game theorist Thomas Schelling.

I also really enjoyed Kevin Houston's fascinating and detailed article The Beatles' magical mystery chord, delving into the mathematics and music behind the opening chord to a hard days night. It's made me want to catch the next talk Kevin gives on the maths of autotune!

A very mathematical snowflake

For a real treat, why not spend a long lunch hour watching Terry Tao's talk The Cosmic Distance Ladder. This forms part of one of Mike Lawler's blog post about the lecture, where he discuses how the radius of the Earth and the radius of the Moon's orbit were measured, and how Tao's lecture was an incredible opportunity to show some fun and exciting maths to children. Lawler said in his submission: "Amazingly this lecture by Tao on youtube has fewer than 2,000 views right now. I wish that more people knew about it (kids especially) as it is such an amazing opportunity to learn from one of the greatest mathematicians living today." We heartily agree! (And you can find out more on Tao's work here.)

I, and any other people madly manufacturing paper snowflakes for decorating school halls and homes, must thank Mr Reid for his excellent post A public service announcement regarding paper snowflakes. Really, if you're in the middle of cutting a million and one of those suckers out you need to read this, it will make it all much better!

And here are some more treats to while away your paper-snowflake-cutting-time…

That's it for this Carnival, we hope you enjoyed the show! You can find the last Carnival at CavMaths and the details for all past and future Carnivals, as well as how to be involved at Aperiodical. Don't forget to submit ideas for January's Carnival, hosted by Andrew at AndrewT.net.

(Hint for the division rule: A number with digits abcd=ax1000+bx100+cx10+d = ax(1+999)+bx(1+99)+c(1+9)+d…)

For a feel of the book read our short introduction to the maths behind their beautiful images. In the book itself, as well as pages of stunning images, you'll find instructions for how to create the images yourself, as well as detailed explanation of the mathematics involved. One of the most compelling ideas is the way these contained images can represent the infinite. In many eastern philosophies, especially Buddhist, this idea of the infinite appearing from copies within copies is pervasive: "In a single atom, great and small lands, as many as atoms." This concept was so exactly reflected in the mathematics of their pictures that it inspired the title of the book, taken from the ancient Buddhist myth of Indra's Web:

In the heaven of the great god Indra is said to be a vast and shimmering net, finer than a spider's web, stretching to the outermost reaches of space. Strung at each intersection of its diaphanous threads is a reflecting pearl. Since the net is infinite in extent, the pearls are infinite in number. In the glistening surface of each pearl are reflected all the other pearls, even those in the furthest corners of the heavens. In each reflection, again are reflected all the infinitely many other pearls, so that by this process, reflections of reflections continue without end.

Why do diamonds sparkle? Why is the shower the best place to sing? Where is the 4th dimension in Dali's paintings? If you don't know the answers to these questions, or perhaps
didn't even know you didn't know these interesting facts, then John D. Barrow's book will help. Its title, 100 essential things you didn't know you didn't know about maths and the arts, explains what's in it and you can a taster for Barrow's writing in thisPlus article. You can be sure it's good though, after all this is Barrow's 22nd book!

If you've got maths friends then you might have noticed that a large subset of them are also Simpsons fans. And there's a good reason for that: as Simon Singh's book The Simpsons and their mathematical secrets explains, a large number of Simpsons writers are mathematicians. The book explores the maths that has been hidden (and not so hidden) throughout the series and will make a great present for maths/Simpsons nerds.